1. Field of the Invention
The invention is directed to low gloss blends of aromatic carbonate polymers with graft polymer additives based on EPDM rubber.
2. Brief Description of the Prior Art
Aromatic carbonate polymers, such as polycarbonates, have become major commercial thermoplastics in view of their excellent physical properties. While the impact strength is good in comparison with many other plastics, it can be further improved by addition of impact modifying resins.
Polycarbonates as well as such blends with impact modifiers tend to have high gloss characteristics. For some purposes, this is desirable. However, a matte or non-glossy surface is often preferable for products such as housings for computer terminals, typewriters, miscellaneous electrical appliances and certain automotive parts. There is substantial commercial need for molding resin compositions which are non-glossy.
Elimination of gloss by surface embossing has been practiced but requires a separate step and adds cost. Moreover, subsequent abrasion may remove the embossed matte surface and cause the gloss to reappear.
The addition of a delustering agent such as finely-divided silica, silicate, alumina, or other mineral filler has a seriously detrimental effect on the physical properties of the molded article such as impact strength. Attempts to add a polymeric delustering agent frequently also have deleterious effects on physical properties, not only impact strength but also on weldline strength, heat distortion temperature, and other important properties. This is not surprising because the low gloss property arises from formation of domains or phases of optical dimensions, and the requirements for the separation of the components to form such phases tends to work against optimal strength properties and in the extreme can even cause delamination and weak weldlines. The weldlines, also known as knit lines are the surfaces of juncture between the flows coming in from different gates; ideally, these should not be weak places in molded articles using multi-gate (such as double gate) molds.
The applications for which low gloss is desired happen to be mostly ones which also demand good strength properties including strong weldlines. The present invention provides means for achieving low gloss with good impact and strong weldlines in impact modified polycarbonate blends.